An event trigger fires whenever the event with which it is
associated occurs in the database in which it is defined.
Currently, the only supported events are ddl_command_start, ddl_command_end and sql_drop. Support for additional events may be
added in future releases.

The ddl_command_start event occurs
just before the execution of a CREATE,
ALTER, or DROP
command. No check whether the affected object exists or doesn't
exist is performed before the event trigger fires. As an
exception, however, this event does not occur for DDL commands
targeting shared objects — databases, roles, and tablespaces — or
for commands targeting event triggers themselves. The event
trigger mechanism does not support these object types. ddl_command_start also occurs just before the
execution of a SELECT INTO command,
since this is equivalent to CREATE TABLE
AS.

The ddl_command_end event occurs just
after the execution of this same set of commands.

The sql_drop event occurs just before
the ddl_command_end event trigger for
any operation that drops database objects. To list the objects
that have been dropped, use the set-returning function pg_event_trigger_dropped_objects() from the
sql_drop event trigger code (see
Section 9.28). Note
that the trigger is executed after the objects have been deleted
from the system catalogs, so it's not possible to look them up
anymore.

Event triggers (like other functions) cannot be executed in an
aborted transaction. Thus, if a DDL command fails with an error,
any associated ddl_command_end triggers
will not be executed. Conversely, if a ddl_command_start trigger fails with an error, no
further event triggers will fire, and no attempt will be made to
execute the command itself. Similarly, if a ddl_command_end trigger fails with an error, the
effects of the DDL statement will be rolled back, just as they
would be in any other case where the containing transaction
aborts.

For a complete list of commands supported by the event trigger
mechanism, see Section
37.2.

Event triggers are created using the command CREATE EVENT TRIGGER. In order
to create an event trigger, you must first create a function with
the special return type event_trigger.
This function need not (and may not) return a value; the return
type serves merely as a signal that the function is to be invoked
as an event trigger.

If more than one event trigger is defined for a particular
event, they will fire in alphabetical order by trigger name.

A trigger definition can also specify a WHEN condition so that, for example, a ddl_command_start trigger can be fired only for
particular commands which the user wishes to intercept. A common
use of such triggers is to restrict the range of DDL operations
which users may perform.